Coatings for metal interconnects to reduce SOFC degradation

ABSTRACT

A method of coating an interconnect for a solid oxide fuel cell includes providing an interconnect including Cr and Fe, and coating an air side of the interconnect with a manganese cobalt oxide spinel coating using a plasma spray process.

FIELD

The present invention is directed to fuel cell stacks, specifically tointerconnects and methods of making interconnects for fuel cell stacks.

BACKGROUND

A typical solid oxide fuel cell stack includes multiple fuel cellsseparated by metallic interconnects (IC) which provide both electricalconnection between adjacent cells in the stack and channels for deliveryand removal of fuel and oxidant. The metallic interconnects are commonlycomposed of chromium containing alloys which retain its strength and isdimensionally stable at typical solid oxide fuel cell (SOFC) operatingconditions, e.g. 700-900 C. However, during operation of the SOFCschromium in the alloys reacts with oxygen and forms chromia, resultingin degradation of the adjacent SOFCs.

Two of the major degradation mechanisms affecting SOFCs are directlylinked to chromia formation of the metallic interconnect component: i)ohmic resistance due to the formation of native chromia (i.e., chromiumoxide, which can be expressed as Cr₂O₃) on the interconnect, and ii)chromium poisoning of the cathode. The chromium containing alloy formsthe native oxide of chromium oxide (Cr₂O₃) at SOFC operatingtemperatures (700-900 C) in both air and wet fuel atmospheres. AlthoughCr₂O₃ is electrically conductive, the conductivity of this material atSOFC operating temperatures (700-900 C) is relatively low, with valueson the order of 0.01 S/cm at 850 C (versus 7.9×10⁴ S/cm for Cr metal).The chromium oxide layer grows in thickness on the surfaces of theinterconnect with time and thus the ohmic resistance due to this oxidelayer increases with time.

The second degradation mechanism is known as chromium poisoning of thecathode. During fuel cell operation, ambient air (humid air) flows overthe air (cathode) side of the interconnect and wet fuel flows over thefuel (anode) side of the interconnect. At SOFC operating temperaturesand in the presence of humid air on the cathode side, chromium on thesurface of the Cr₂O₃ layer on the interconnect reacts with water andevaporates in the form of the gaseous species chromium oxide hydroxide,Cr₂O₂(OH)₂. The chromium oxide hydroxide species transports in vaporform from the interconnect surface to the cathode electrode of the fuelcell where it deposits in the solid form as chromia, Cr₂O₃. The Cr₂O₃deposits on and in (e.g., via grain boundary diffusion) the SOFCcathodes and/or reacts with the cathode (e.g. to form a Cr—Mn spinel),resulting in significant performance degradation of the cathodeelectrode. Typical SOFC cathode materials, such as perovskite materials,(e.g., lanthanum strontium manganate (“LSM”), LSC, LSCF, and LSF) areparticularly vulnerable to chromium oxide degradation.

SUMMARY

An embodiment relates to a method of coating an interconnect for a solidoxide fuel cell, comprising providing an interconnect substratecomprising Cr and Fe, and coating an air side of the interconnectsubstrate with a manganese cobalt oxide spinel coating using a plasmaspray process.

In an aspect, the method may further include placing the coatedinterconnect substrate into a solid oxide fuel cell stack. In a furtheraspect, the method may also include removing a chromia layer from theair side of the interconnect substrate prior to the step of coating suchthat the spinel coating is formed directly on the chromium-iron alloysurface of the air side of the interconnect substrate that is not coatedwith chromia; and forming a manganese-cobalt-chromium intermediatespinel layer between the spinel coating and the chromium-iron alloysurface of the air side of the interconnect substrate by reacting thespinel coating with the chromium-iron alloy surface of the air side ofthe interconnect substrate at an elevated temperature after the step ofplacing.

Another embodiment relates to a coated interconnect for a solid oxidefuel cell, comprising an interconnect substrate comprising at least 70weight percent chromium, and a manganese cobalt oxide spinel coatingformed over an air side of the interconnect substrate, wherein thespinel comprises a Co:Mn atomic ratio of at least 1:3.

Another embodiment relates to a coated interconnect for a solid oxidefuel cell, comprising an interconnect substrate comprising iron andchromium, a manganese cobalt oxide spinel coating formed over an airside of the interconnect substrate, a manganese-cobalt-chromiumintermediate spinel layer located between the spinel coating and the airside of the interconnect substrate, and a perovskite layer located overthe spinel coating.

Another embodiment relates to a coated interconnect for a solid oxidefuel cell. The interconnect includes an interconnect substrate includingiron and chromium and a composite spinel and perovskite coating formedover an air side of the interconnect substrate. In an aspect, the spinelphase comprises manganese cobalt oxide spinel and the perovskite phasecomprises lanthanum strontium manganate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side schematic illustration of an embodiment of aninterconnect with a spinel MCO coating and an underlying interfacialoxide. FIG. 1B illustrates a micrograph of an as-deposited APSMn_(1.5)Co_(1.5)O₄ coating according to an embodiment.

FIG. 2 illustrates a plot of voltage versus time of SOFC stacks testedin dry air comparing degradation rates of repeat elements with LSM toMn_(1.5)Co_(1.5)O₄ spinel interconnect coatings.

FIG. 3 illustrates a plot of voltage versus time of SOFC stacks testedin wet air comparing degradation rates of repeat elements with LSM,Mn_(1.5)Co_(1.5)O₄ spinel, and double layer LSM-Mn_(1.5)Co_(1.5)O₄interconnect coatings.

FIGS. 4A and 4B illustrate micrographs of Mn_(1.5)Co_(1.5)O₄ spinelcoating and LSM perovskite coating, respectively after 1000 hrs ofoperation in SOFC stack.

FIG. 5A is a side schematic illustration of an embodiment of aninterconnect with a bilayer coating. FIG. 5B is a side schematicillustration of an embodiment of a solid oxide fuel cell stack whichincludes an interconnect with a bilayer coating on its ribs.

FIG. 6 is a micrograph illustrating an embodiment of an interconnectwith a composite LSM-MCO coating.

DETAILED DESCRIPTION

Interconnects may be coated with a single-phase oxide coating consistingof either a perovskite or spinel structure to decrease the growth rateof the native chromium oxide layer and suppress the evaporation of thechromium vapor species. Two common candidates are strontium-dopedlanthanum manganate (LSM) and manganese cobaltite (MCO), respectively.LSM is an excellent candidate because of its high electricalconductivity at elevated temperatures (i.e., it does not add substantialohmic resistance), low oxygen conductivity which suppresses oxide growthunderneath it on the IC, and low cation conductivity which suppressessolid state diffusion of Cr through the coating.

Alternatively, MCO is a good candidate for IC coating because it forms aMn and Co-doped Cr-based oxide beneath the coating on the surface of theIC. The Mn and Co-doped Cr-based oxide has higher electricalconductivity than the native chromia layer. Further, this oxide sintersduring high-temperature operation which prevents crack formation or thecreation of escape pathways for Cr.

A first embodiment includes applying a coating to an interconnect todecrease the growth rate of the native chromium oxide layer and suppressthe evaporation of the chromium vapor species. In one aspect of thisembodiment, the coating is a Mn—Co based spinel material (“MCO”).

FIG. 1A illustrates the MCO spinel layer 102 over an air side of achromium containing interconnect 100. The MCO spinel layer 102 may havethe following formula (M1, M2)₃O_(4±0.1) where M1 comprises at least 70atomic percent, such as 70-100 at % manganese, and M2 comprises at least70 atomic percent, such as 70-100 at % cobalt. M1 and/or M2 may containadditional elements, as will be described with respect to the subsequentembodiments below. The MCO spinel encompasses the compositional rangefrom M1₂M2₁O_(4±1) to M2₂M1₁O_(4±0.1).

In the first embodiment, M1 consists of Mn (and unavoidable impurities,if any) and M2 consists of Co (and unavoidable impurities, if any) andthe spinel is stoichiometric (i.e., the metal to oxygen atomic ratio is3:4). In the first embodiment, the MCO spinel encompasses thecompositional range from Mn₂CoO₄ to Co₂MnO₄. That is, any spinel havingthe composition Mn_(2−x)Co_(1+x)O₄ (0≤x≤1) or written asz(Mn₃O₄)+(1−z)(Co₃O₄), where (⅓≤z≤⅔) or written as (Mn, Co)₃O₄ may beused.

Preferably the spinel composition contains at least 25 atomic percent ofcobalt oxide, such as 25 to 60 atomic percent cobalt oxide. Another wayto phrase this is that the atomic ratio of Co to Mn in the spinel ispreferably at least 1:3, such as 1:3 to 6:4, preferably 1:1. Thus, thepreferred but non-limiting spinel composition is Mn_(1.5)Co_(1.5)O₄which comprises 50 atomic percent manganese oxide and fifty atomicpercent cobalt oxide. The MCO coating 102 may have any suitablethickness, such as 20 to 100 microns, preferably greater than 20microns, such as 25 to 40 microns.

Any suitable chromium containing interconnect substrate 100 may be used.Preferably, the substrate 100 is a chromium based alloy, such as analloy containing at least 70 weight percent chromium, for example 92 to97 weight percent chromium, 3 to 7 weight percent iron, and optionally 0to 1 weight percent of yttrium, yttria, other alloying elements and/orunavoidable impurities. Preferably, the substrate 100 comprises theso-called CrF alloy (e.g., 95 weight percent Cr and 5 weight percentFe). The alloy may be oxidized on its surface and/or throughout itsvolume, such that the substrate contains a chromium and/or iron oxidelayer on its surface or oxide regions in its volume. However, othersuitable substrate 100 materials may be used instead, such as nicrofer,Inconel 600 or X750, Crofer 22 APU or other chromium containingstainless steels.

As shown in FIG. 1A, the interconnect may contain an intermediate (i.e.,interfacial) oxide layer 106 between the substrate 100 and the MCOspinel coating 102. The intermediate oxide layer 106 may comprise anative chromia layer on the CrF interconnect substrate 100 and/or theintermediate oxide layer 106 may comprise an intermediate spinel layercontaining Cr, Mn, O and optionally Co. For example, the intermediateoxide layer 106 may comprise a (Mn, Cr, Co)₃O₄ spinel layer formed byreacting the MCO coating 102 with the chromium containing substrate 100during the SOFC stack high temperature operation or stack annealing tomelt the seals or reduce a nickel oxide in the cermet SOFC anodeelectrodes to nickel, as will be described below. The intermediate layer106, if present, may have a thickness of 5 microns or less, such as 1 to5 microns.

The MCO coating 102 is deposited on the interconnect substrate 100 usingany suitable deposition method. Preferably, the coating 102 is depositedby a plasma spray process, such as an air plasma spray (APS) process. Ina plasma spray process, a feedstock powder is introduced into a plasmajet or spray, emanating from a plasma source, such as a plasma torch.The feedstock powder is melted in the plasma jet (where the temperatureis over 8,000K) and propelled towards the interconnect substrate 100.There, the molten droplets flatten, rapidly solidify and form the MCOspinel coating 102. Preferably, the feedstock powder comprises MCOpowder having the same composition as the coating 102. However, metal(e.g., Mn, Co or Mn—Co alloy) powder may be used instead andsubsequently oxidized to form the MCO spinel coating 102.

The plasma may be generated by either direct current (e.g., electric arcDC plasma) or by induction (e.g., by providing the plasma jet through acenter of an induction coil while a RF alternating current passesthrough the coil). The plasma may comprise a gas stabilized plasma(e.g., argon, helium, etc,). Preferably, the plasma spraying is airplasma spraying (APS) which is performed in ambient air. Alternatively,a controlled atmosphere plasma spraying (CAPS) method may be used whichis performed in a closed chamber, which is either filled with an inertgas or evacuated.

Preferably, the native oxide layer is removed from the interconnectsubstrate 100 prior to the deposition of the coating. For example, thenative chromia layer may be removed from the CrF substrate 100 bygrinding, polishing, grit blasting, etching or other suitable methodsbefore deposition of the MCO coating 102, such that the native chromiadoes not substantially reform prior to MCO coating deposition. FIG. 1Bis a micrograph illustrating an as-deposited Mn_(1.5)Co_(1.5)O₄ coating102 on an interconnect 100 after the native chromia layer is removed.The W_(1.5)Co_(1.5)O₄ coating 102 exhibits good density with someporosity and microcracking.

Planar SOFC stacks containing some interconnects coated with theMn_(1.5)Co_(1.5)O₄ coating and some interconnects with an LSM coatingwere tested to give a head-to-head comparison of the coatings. Theresults of these tests are illustrated in FIGS. 2 and 3. FIGS. 2 and 3are plots of the voltage verses time for two different SOFC stacks thatboth contain Mn_(1.5)Co_(1.5)O₄ and LSM coated interconnects 100. Theresults shown in FIG. 2 are for a stack tested with dry air while theresults shown in FIG. 3 are for a stack tested with humid air. In bothcases, the degradation rates of the repeat layers with theMn_(1.5)Co₁₅O₄ coatings are significantly lower than those with the LSMcoatings.

In the case of dry air testing (FIG. 2), the Mn_(1.5)Co_(1.5)O₄ coatingexhibited a lower ASR degradation relative to the LSM coating. The lowerresistivity of the Mn and Co doped Cr spinel layer 106 results in lowerdegradation of the interconnects and the adjacent SOFC cathodeelectrodes for Mn_(1.5)Co_(1.5)O₄ coated interconnects relative to LSMcoated interconnects (and their adjacent SOFC cathode electrodes). Theresistance of this intermediate chromium containing oxide layer isdependent on both the thickness of the intermediate oxide layer andelectrical conductivity of the oxide layer. The coating layer affectsthe resistance of the oxide layer in two ways, by: i) reducing thegrowth rate and thus thickness of oxide layer at a given time, and ii)reacting with the oxide layer to produce a secondary oxide phase thathas a different composition and conductivity.

The MCO coating 102 acts as a barrier layer suppressing the diffusion ofoxygen from the air stream to the intermediate oxide layer 106 on theinterconnect substrate 100. This in turn reduces the growth rate of thenative chromia layer and/or the intermediate spinel layer 106. Coatingsthat are effective in reducing oxygen transport from the air stream tothe native oxide include materials that exhibit low oxygen diffusivity(solid state diffusion of oxide ions), such as spinel phases. Thephysical characteristics of a good protective coating include havinghigh density, low connected porosity, no microcracking, and completecoverage of the interconnect.

The coating 102 also affects the resistance of the native oxide byinterdiffusion and the formation of secondary phases. The oxide layerthat forms on an uncoated CrF interconnect is the native oxide, Cr₂O₃.This oxide exhibits conductivity on the order of 0.01 S/cm at 850 C.However, with a coating on the interconnect, a reaction occurs betweenthe native Cr₂O₃ oxide and the coating material. This reaction resultsin the formation of a reaction zone oxide layer which has a conductivitydifferent from either the Cr₂O₃ native oxide or the coating material.

In the case of a CrF interconnect coated with LSM, the reaction zoneoxide layer that forms is in the spinel family (Mn, Cr)₃O₄. Theconductivity of the (Mn, Cr)₃O₄ spinel is dependent on the composition,with examples given as follows: MnCr₂O₄: 0.003 S/cm, Mn_(1.2)Cr_(1.8)O₄0.02 S/cm and Mn_(1.5)Cr_(1.5)O₄ 0.07 S/cm at 800 C.

Generally, the conductivity of the (Mn, Cr)₃O₄ spinels are slightlybetter than the native Cr₂O₃. However, the thickness of the reactionzone oxide can be thicker than the native oxide. Thus the total ohmicresistance can be larger. With a Mn_(1.5)Co_(1.5)O₄ spinel coating onCrF materials, the reaction zone intermediate oxide layer 106 includes a(Mn, Cr, Co)₃O₄ spinel phase. Layer 106 may comprise 60 to 100 volumepercent of the (Mn, Cr, Co)₃O₄ spinel phase, with the balance (if any)being chromia or other phases. The conductivity of the cobalt containing(Mn, Cr, Co)₃O₄ family of spinels is considerably higher than that ofthe (Mn, Cr)₃O₄ spinels as given by the examples: MnCo₂O₄: 36 S/cm,CoCr₂O₄: 7 S/cm, and CoMn₂O₄: 6 S/cm. The higher conductivity of thereaction zone oxide created with the Mn_(1.5)Co_(1.5)O₄ spinel coating(which preferably has an electrical conductivity of at least 20 S/cm,such as at least 38 S/cm) on CrF results in lower ohmic resistancelosses from this interface and thus lower SOFC performance degradationwith time.

In wet air atmospheres and at SOFC operating temperatures, theevaporation rates of chromium from the surface of the interconnect arerelatively high. Therefore containment by a coating is preferable. Theresults of the SOFC stack tested in humid air are illustrated in FIG. 3.These results also show that the repeat layers with theMn_(1.5)Co_(1.5)O₄ spinel coating exhibit lower degradation as comparedto the repeat layers with LSM coating.

The lower degradation with the Mn_(1.5)Co_(1.5)O₄ spinel coating may beattributed to both i) the lower ohmic resistance of the reaction zoneoxide layer, and ii) the reduction in the rate of chromium evaporation.FIG. 4A shows a micrograph of the Mn_(1.5)Co_(1.5)O₄ coating 102 on aCrF substrate 100 after operating in a SOFC stack (as described withrespect to FIG. 5B below) at elevated operating temperature for 1000hrs. FIG. 4B shows a micrograph of an LSM coating 104 on the CrFsubstrate 100 in a SOFC stack operated at elevated operating temperaturefor 1000 hrs. As evident in the micrographs, the Mn_(1.5)Co_(1.5)O₄coating 102 is dense, has no open porosity, and does not havemicrocracks 103. This is in contrast to the LSM coating 104 which hasmicrocracking after SOFC operation.

The LSM coating 104 tends to sinter during SOFC operation, leading tothe formation of microcracks 103 which can allow chromium vaportransport through the coating. Comparison of FIG. 1B with FIG. 4Aindicates that even though the as-deposited APS coating ofMn_(1.5)Co_(1.5)O₄ shown in FIG. 1B may contain some microcracking andfissures, after a period of time at SOFC operating temperatures (e.g.,700 to 900 C), the MCO coating appears to densify in such a way to healand eliminate connected microcracks, as shown in FIG. 4B. Likewise, theintermediate chromium containing spinel layer 106 is formed by reactionbetween the MCO coating 102 and the CrF interconnect substrate 100during stack fabrication annealing and/or during stack operation atelevated temperature.

A second embodiment of the invention includes doping the spinel powder,e.g. (Mn, Co)₃O₄, with Cu to reduce the melting temperature of thespinel. The lowered melting temperature improves (increases) the coatingdensity upon deposition with APS and increases the conductivity ofreaction zone oxide. The improvement in the density of the coating dueto the lower melting temperature can occur during APS deposition andduring operation at SOFC temperature for extended periods of time.

The addition of Cu to the spinel layer has an additional advantage. TheCu doping of the spinel, such as (Mn, Co)₃O₄, may result in higherelectrical conductivity of the base spinel phase as well as any reactionzone oxides that form between the spinel and the native Cr₂O₃ oxide.Examples of electrical conductivities of oxides from the (Mn, Co, Cu,Cr)₃O₄ family include: CuCr₂O₄: 0.4 S/cm at 800 C, Cu_(1.3)Mn_(1.7)O₄:225 S/cm at 750 C, and CuMn₂O₄: 40 S/cm at 800 C.

The spinel family of materials has the general formula AB₂O₄. Thesematerials may form an octahedral or cubic crystal structure depending onthe elements occupying the A and B sites. Further, depending on thedoping conditions, the doped copper may occupy either the A site, the Bsite or a combination of the A and B sites. Generally, Cu prefers to gointo B site. When the A element is Mn, the B element is Co, and thespinel is doped with Cu, the spinel family may be described with thegeneral formula (Mn, Co, Cu)₃O₄. More specifically, the spinel familymay be described with the following formulas depending on location ofthe Cu alloying element:Mn_(2−x−y)Co_(1+x)Cu_(y)O₄(0≤x≤1), (0≤y≤0.3) if Cu goes in A site  (1)Mn_(2−x)Co_(1+x−y)Cu_(y)O₄(0≤x≤1), (0≤y≤0.3) if Cu goes in B site  (2)Mn_(2−x−y/2)Co_(1+x−y/2)Cu_(y)O₄(0≤x≤1), (0≤y≤0.3) if Cu goes equally inboth A and B site.  (3)

Specific (Mn, Co, Cu)₃O₄ compositions include, but are not limited to,Mn_(1.5)Co_(1.2)Cu_(0.3)O₄, Mn_(1.5)Co_(1.4)Cu_(0.1)O₄;Mn₂Co_(0.8)Cu_(0.2)O₄ and Co₂Mn_(0.8)Cu_(0.2)O₄. Additional compositionsinclude Mn₂Co_(1−y)Cu_(y)O₄, where (0≤y≤0.3), if Cu goes in B site.These composition may also be written, (Mn₂O₃)+(1−z)(CoO)+z(CuO), where(0≤z≤0.3). Other compositions include Co₂Mn_(1−y)Cu_(y)O₄ where(0≤y≤0.3) if Cu goes in B site. These composition may also be written,(Co₂O₃)+(1−z)(MnO)+z(CuO) where (0≤z≤0.3). In one preferred Mn, Cospinel composition, the Mn/Co ratio is 1.5/1.5, e.g. Mn_(1.5)Co_(1.5)O₄.When B site doped with Cu, preferred compositions includeMn_(1.5)Co_(1.5−y)Cu_(y)O₄, where (0≤y≤0.3).

In a third embodiment, (Mn, Co)₃O₄ or (Mn, Co, Cu)₃O₄ spinel familiesare doped with one or more single valence species. That is, one or morespecies that only have one valence state. Doping with single valencespecies reduces cation transport at high temperature and thus reducesthe thickness of the intermediate oxide layer 106. The primary ionictransport mechanism in spinels is through cation diffusion via cationvacancies in the lattice structure. In spinels with multivalent speciesM^(2+/3+), such as Mn^(3+/4+) and Co^(2+/3+), cation vacancies aregenerated when M species are oxidized from lower to higher valancestates to maintain local charge neutrality. The introduction of a singlevalence species typically decreases the amount of cation vacancies anddecreases the amount of interdiffusion between the spinel coating 102and the native Cr₂O₃ oxide or the CrF substrate 100. In this manner, theamount of the intermediate oxide layer 106 that forms is decreased.Examples of single valence species that may be introduced into thespinel coating include Y³⁺, Al³⁺, Mg²⁺ and/or Zn²⁺ metals. In an aspect,the spinel coating has a composition of (Mn, Co, M)₃O₄, where M=Y, Al,Mg, or Zn. For example, if M=Al doped in the A position, then the spinelcompositions may include Mn_(2−y)Al_(y)CoO₄ (0≤y≤0.3) or(1−z)(Mn₂O₃)+z(Al₂O₃)+CoO, where (0≤z≤0.15).

In a fourth embodiment, a second phase is added to a (Mn, Co)₃O₄ spinelto act as a getter for impurities, such as sulfur and silicon. In thismanner, the adhesion of the coating to the CrF interconnect substrate100 may be improved. For example, metal oxide phases, such as non-spinelmetal oxides, for example, Al₂O₃, Y₂O₃, or TiO₂ may be added to thespinel phase of the coating 102 as a second phase. In one aspect, whenthe metal oxide phase is alumina, the coating composition may be(1−x)(Mn, Co)₃O₄ and x(Al₂O₃), where (0≤x≤0.02). In this case, the Al₂O₃primarily exists as a second phase and not as a doping agent in thespinel structure. During deposition and at SOFC operation temperatures,however, some interdiffusion may occur. In this case, aluminum, yttriumor titanium doping of the spinel phase will occur.

In a fifth embodiment illustrated in FIG. 5A, the protective coating isa bi-layer film composed of an MCO spinel coating 102 over theinterconnect substrate 100 and a perovskite layer 104 over the MCOspinel coating. The perovskite layer may comprise any suitableperovskite layer described above, such as LSM. LSM may have thefollowing formula: La_(1−x)Sr_(x)MnO₃ (LSM), where 0.1≤x≤0.3, such as0.1≤x≤0.2. The spinel coating 102 is deposited first and is in directcontact with the interconnect substrate 100 (if the native chromia layeris removed from the substrate) or the native chromia layer on thesubstrate 100. The perovskite layer 104 is then deposited on top of thespinel coating 102. The intermediate oxide layer 106 may be formed byreaction between the spinel coating and the interconnect substrateduring subsequent interconnect or stack annealing or operation. Thebi-layer coating may decrease the degradation of the adjacent SOFC inthree ways. First, the spinel coating 102 provides Mn and Co elements tothe intermediate oxide layer 106, thereby decreasing the resistance ofthe intermediate chromium containing oxide layer 106. Second, the spinelcoating 102 prevents direct interaction between the perovskite layer104, such as LSM, and the chromium containing intermediate oxide layer106 which can lead to the formation of unwanted and resistive secondaryphases. Third, the top perovskite layer 104 is a second barrier layerthat decreases the transport of oxygen to the intermediate oxide layer106. The top perovskite layer 104 thus reduces the growth rate and thethickness of a native chromia on the substrate 100. The top perovskitelayer 104 also reduces the amount of chromium transport (via solid stateor gas phase) from the metallic interconnect substrate 100 to thecathode electrode of the adjacent SOFC in the stack.

In another aspect of the fifth embodiment shown in FIG. 5B, the MCOcoating or layer 102 is formed over the entire air side or surface ofthe interconnect 9, while the perovskite layer 104 is formed on the MCOcoating 102 only on the ribs or ridges of the interconnect 9, but not inthe air flow channels or passages in the interconnect 9.

An example of a solid oxide fuel cell (SOFC) stack is illustrated inFIG. 5B. Each SOFC 1 comprises a cathode electrode 7, a solid oxideelectrolyte 5, and an anode electrode 3. Fuel cell stacks are frequentlybuilt from a multiplicity of SOFC's 1 in the form of planar elements orother geometries. Fuel and air is provided to the electrochemicallyactive surfaces of the anode 3 and cathode 7 electrodes, respectively.The interconnect 9 containing gas flow passages or channels 8 betweenribs 10, separates the individual cells in the stack. The interconnect 9electrically connects the anode or fuel electrode 3 of one cell 1 to thecathode or air electrode 7 of the adjacent cell 1. The interconnect 9separates fuel, such as a hydrocarbon fuel, flowing in the fuel channels8 between ribs 10 on the fuel side of the interconnect to the fuelelectrode (i.e. anode 3) of one cell 1 in the stack, from oxidant, suchas air, flowing in the air channels 8 between ribs 10 on the air side ofthe interconnect to the air electrode (i.e. cathode 7) of an adjacentcell 1 in the stack. At either end of the stack, there may be an air endplate or fuel end plate (not shown) for providing air or fuel,respectively, to the end electrode. FIG. 5B shows that the lower SOFC 1is located between two interconnects 9.

As shown in FIG. 5B, the MCO spinel coating 102 is located over the ribs10 and the air channels 8 on the air side of the interconnects 9 facingthe adjacent SOFC 1 cathode 7 in the stack. The perovskite layer 104(e.g., LSM) is located over the MCO coating 102 only over the rib 10areas, but not in the air channel 8 areas. This allows the perovskite(e.g., LSM) layer 104 to contact the same or similar perovskite (e.g.,LSM) cathode 7, without coating the entire air side of the interconnect9 with the perovskite layer 104. The intermediate oxide layer (not shownin FIG. 5B) above may be formed between the CrF interconnect 9 substrate100 and the MCO spinel coating 102 after stack annealing and/oroperation.

The sixth embodiment of the invention provides a composite perovskiteand spinel coating rather than the bilayer spinel and perovskitecoating. FIG. 6 is a micrograph illustrating the interconnect with acomposite LSM-MCO coating. The composite LSM/MCO coating 110 accordingto this embodiment is designed to utilize the best features of each ofthese individual coatings discussed above. The composite coating 110illustrated in FIG. 6 comprises 40 wt % MCO and 60 wt % LSM and isconditioned (two day cycle at 850° C.) in a SOFC stack. The light phase112 is LSM while the dark phase 114 is MCO. Thus, the LSM and MCO phasesare present as distinct regions in the composite coating. Withoutwishing to be bound by a particular theory, it is believed that the MCOphase may form plate-like or pancake-like (e.g., longer than thicker)structures 114 in the LSM phase matrix 112. However, the structure maybe different for different compositions and/or deposition methods of thecomposite coating 110.

The presence of crack-healing pancake-like MCO structures within thecomposite coating 110 suppresses Cr evaporation through cracks generatedin the LSM. The presence of LSM stabilizes the composite LSM/MCO coatingin reducing atmospheres such that spallation does not occur and coatingintegrity is maintained. Preferably, the MCO content of the compositecoating 110 is sufficiently high to form the Mn—Cr—Co oxide (e.g.,spinel 106) scale on the IC, which provides lower ohmic resistancecompared to a single-phase LSM coating, which may only form a MnCr oxidespinel on the interconnect surface. As can be seen in FIG. 6, thecomposite coating 110 does not exhibit any cracking or spalling aftertwo days of conditioning at 850° C. in the SOFC stack.

The composition of the composite coating 110 may be any ratio of LSM:MCOas long as there is a mix of the two materials (not a bi-layer coating).For example, the perovskite to spinel (e.g., LSM:MCO) weight ratio mayrange between 20:80 and 90:10, such as 50:50 to 80:20. The compositionof the individual LSM and MCO materials in the composite coating 110 mayvary as described above and may have any level or ratio of non-oxygenconstituents, and may include other phases besides the pervoskite andspinel phase and/or other elements besides Mn, Co, La, Sr and O. Forexample, the spinel phase 114 of the coating 110 may compriseMn_(2−x)Co_(1+x)O₄, where 0≤x≤1 and the perovskite phase 112 maycomprise La_(1−x)Sr_(x)MnO₃ (LSM), where 0.1≤x≤0.3, such as 0.1≤x≤0.

The composite coating 110 may be deposited on the interconnect using anydeposition method, such as, but not limited to APS. Preferably, theperovskite and spinel are deposited together in one step. For example,APS feedstock powder provided into the plasma in the APS process maycomprise a mixture of LSM and MCO powder having the same weight ratio asthat desired for the coating 110.

The microstructure, thickness, or any other physical property of thecoating may vary and can be of any form. However, a dense coating ispreferred. The composite coating 110 may be deposited on any location onthe interconnect. That is, the composite coating is not limited to anyspecific portion of the interconnect, but is preferred to be depositedon the cathode side of the interconnect.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the invention is not so limited. It will occurto those of ordinary skill in the art that various modifications may bemade to the disclosed embodiments and that such modifications areintended to be within the scope of the invention. All of thepublications, patent applications and patents cited herein areincorporated herein by reference in their entirety.

What is claimed is:
 1. A method of coating an interconnect for a solidoxide fuel cell, comprising: providing an interconnect substratecomprising a chromium-iron alloy comprising at least 70 weight percentchromium; removing a native chromia layer from the interconnectsubstrate to expose a chromium-iron alloy surface of the interconnectsubstrate; providing a mixture of manganese cobalt oxide spinelfeedstock powder and strontium-doped lanthanum manganate perovskitefeedstock powder into a plasma to coat an air side of the interconnectsubstrate with a single layer composite coating containingstrontium-doped lanthanum manganate and manganese cobalt oxide directlyon the exposed chromium-iron alloy surface of the interconnect substrateusing a plasma spray process; placing the coated interconnect substrateinto a solid oxide fuel cell stack, such that the single layer compositecoating directly contacts a solid oxide fuel cell of the stack anddirectly contacts the air side of the interconnect substrate, the airside of the interconnect substrate comprising the chromium-iron alloycomprising at least 70 weight percent chromium; and conditioning theinterconnect substrate, having on its air side only the single layercomposite coating, in the solid oxide fuel cell stack, to form amanganese-cobalt-chromium intermediate spinel layer between thecomposite coating and the chromium-iron alloy surface of the air side ofthe interconnect substrate, the intermediate spinel layer comprising a(Mn, Cr, Co)₃O₄ spinel phase and the composite coating comprising astrontium-doped lanthanum manganate perovskite phase and a manganesecobalt oxide spinel phase; wherein: forming themanganese-cobalt-chromium intermediate spinel layer during the step ofconditioning comprises forming the intermediate spinel layer by reactingthe single layer composite coating with the chromium-iron alloy surfaceof the air side of the interconnect substrate at an elevated temperatureafter the step of placing; the composite coating comprises distinctregions of the manganese cobalt oxide spinel phase in a matrixcomprising the strontium-doped lanthanum manganate perovskite phase; themanganese cobalt oxide spinel phase regions have plate-like orpancake-like structures; the plate-like or pancake-like structurescomprise crack-healing structures within the composite coating whichsuppress Cr evaporation through cracks generated in the matrixcomprising the strontium-doped lanthanum manganate perovskite phaseduring operation of the solid oxide fuel cell stack; and the matrixcomprising the strontium-doped lanthanum manganate perovskite phasestabilizes the composite coating in reducing atmospheres such thatspallation does not occur during operation of the solid oxide fuel cellstack.
 2. The method of claim 1, wherein the manganese cobalt oxidespinel comprises Mn_(2−x)Co_(1+x)O₄, wherein 0≤x≤1.
 3. The method ofclaim 2, wherein the manganese cobalt oxide spinel comprisesMn_(1.5)Co_(1.5)O₄.
 4. The method of claim 2, wherein: the manganesecobalt oxide spinel comprises a Co:Mn atomic ratio of at least 1:3; andthe interconnect substrate comprises a chromium-iron alloy containing 92to 97 weight percent chromium, 3 to 7 weight percent iron, and 0 to 1weight percent of yttrium or yttria.
 5. The method of claim 1, whereinthe composite coating further comprises a getter.
 6. The method of claim5, wherein the getter comprises at least one of Al₂O₃, Y₂O₃, or TiO₂. 7.The method of claim 1, wherein the intermediate spinel layer comprises60 to 100 volume percent of the (Mn, Cr, Co)₃O₄ spinel phase.
 8. Themethod of claim 1, wherein the composite coating comprises a perovskitephase to spinel phase weight ratio between 20:80 and 90:10.
 9. Themethod of claim 1, wherein the composite coating comprises a perovskitephase to spinel phase weight ratio between 50:50 and 80:20.